Aluminum plating solution, aluminum film, resin structure, porous aluminum object, and porous aluminum object manufacturing method

ABSTRACT

Provided is an aluminum plating solution that has a wide current density range in which aluminum plating can be performed and that has a low solution resistance. The aluminum plating solution contains an aluminum halide, at least one selected from the group consisting of an alkylimidazolium halide, an alkylpyridinium halide, and a urea compound, and an ammonium salt represented by the general formula (1) below. A concentration of the ammonium salt is 1 g/L or more and 45 g/L or less. 
       NR 4   + .X −   General formula (1)
 
     In the general formula, R each represent a hydrogen atom or an alkyl group which may have a side chain and which has 15 or less carbon atoms, X represents a halogen atom, and R may be the same or different from each other.

TECHNICAL FIELD

The present invention relates to an aluminum plating solution, an aluminum film, a resin structure, a porous aluminum object, and a porous aluminum object manufacturing method.

BACKGROUND ART

Porous metal objects having a three-dimensional network structure are used in various applications such as filters, catalyst supports, and battery electrodes. For example, Celmet (manufactured by Sumitomo Electric Industries, Ltd.: registered trademark) formed of a porous nickel object having a three-dimensional network structure (hereinafter referred to as “porous nickel object”) is used as an electrode material of a battery such as a nickel-hydrogen battery or a nickel-cadmium battery. Celmet is a porous metal object having continuous pores and has a feature that the porosity thereof is higher (90% or more) than those of other porous objects such as metal non-woven fabrics.

Such a porous nickel object is obtained by forming a nickel layer on a surface of a skeleton of a resin molded body having continuous pores, such as a urethane foam, then decomposing the foamed resin molded body by heat treatment, and further subjecting nickel to a reduction treatment. The nickel layer is formed by performing a conductive treatment by coating the surface of the skeleton of the foamed resin molded body with a carbon powder or the like, and subsequently depositing nickel by electroplating.

Similarly to nickel, aluminum also has good features such as electrical conductivity, corrosion resistance, and light weight. In battery applications, for example, an aluminum foil whose surface is coated with an active material such as lithium cobalt oxide is used as a positive electrode of a lithium-ion battery.

In order to improve the capacity of such a positive electrode including aluminum, for example, a porous aluminum object having a three-dimensional network structure that provides an increased surface area of aluminum (hereinafter referred to as “porous aluminum object”) may be used, and pore portions of the porous aluminum object may also be filled with an active material. This is because, by using the porous aluminum object, the active material can be retained even in an electrode having a large thickness, and the utilization ratio of the active material per unit area is improved.

An example of the method for manufacturing the porous aluminum object is a method in which a foamed resin molded body having a three-dimensional network structure is plated with aluminum. Japanese Unexamined Patent Application Publication No. 2012-007233 (PTL 1) describes an invention relating to a capacitor in which a porous aluminum object obtained by this plating method is used as an electrode. According to the method described in PTL 1, a porous resin molded body having a three-dimensional network structure can be uniformly plated with aluminum having a high purity, and thus a porous aluminum object with a high quality can be manufactured.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.     2012-007233

SUMMARY OF INVENTION Technical Problem

In an aluminum plating method using a molten salt, electroplating of aluminum on a porous resin object can be performed at room temperature by using an aluminum plating bath in which an organic chloride salt such as 1-ethyl-3-methylimidazolium chloride (EMIC) or 1-butylpyridinium chloride (BPC) and aluminum chloride (AlCl₃) are mixed. In particular, an AlCl₃-EMIC-based plating solution has good liquid properties and is useful as an aluminum plating solution.

Regarding the aluminum plating solution containing EMIC and AlCl₃, it is known that the higher the concentration of AlCl₃, the wider the current density range in which plating can be performed (Setsuko Takahashi and three others, “Aluminum (Al) electroplating of parts”, Technical report of Nisshin Steel Co., Ltd., 1990, No. 63, pp. 44-51). It is described that AlCl₃ does not dissolve at a concentration of 67 mol % or more, and thus a 67 mol % AlCl₃-33 mol % EMIC aluminum plating solution, which contains the largest amount of AlCl₃, is the most suitable in terms of plating efficiency. On the other hand, it is also known that the electrical conductivity of an ionic liquid prepared by mixing AlCl₃ and EMIC improves with an increase in the concentration of EMIC.

If the electrical conductivity of a plating solution is low, the solution resistance is increased, and the voltage necessary for allowing a certain amount of current to flow increases, resulting in an increase in the electric power cost. Furthermore, with the increase in the voltage, an increase in the liquid temperature due to Joule heat also occurs. Accordingly, equipment for cooling the plating solution is also necessary in order to make the plating conditions constant, and thus the electric power cost further increases. For these reasons, the use of a plating solution that can be used for plating at a high current density and has high productivity increases the electric power cost as a whole.

Accordingly, in view of the problems described above, an object of the present invention is to provide an aluminum plating solution that has a wide current density range in which aluminum plating can be performed and that has a low solution resistance.

Solution to Problem

In order to solve the above problems, the present invention adopts the configuration described below.

Specifically, a porous aluminum object according to an embodiment of the present invention is an aluminum plating solution containing an aluminum halide, at least one selected from the group consisting of an alkylimidazolium halide, an alkylpyridinium halide, and a urea compound, and an ammonium salt represented by a general formula (1) below, in which a concentration of the ammonium salt is 1 g/L or more and 45 g/L or less.

NR₄ ⁺.X⁻  General formula (1)

In the general formula (1), R each represent a hydrogen atom or an alkyl group which may have a side chain and which has 15 or less carbon atoms, X represents a halogen atom, and R may be the same or different from each other.

Advantageous Effects of Invention

According to the present invention, it is possible to provide an aluminum plating solution that has a wide current density range in which aluminum plating can be performed and that has a low solution resistance. Description of Embodiments

First, the content of embodiments of the present invention will be listed and described.

(1) An aluminum plating solution according to an embodiment of the present invention is an aluminum plating solution containing an aluminum halide, at least one selected from the group consisting of an alkylimidazolium halide, an alkylpyridinium halide, and a urea compound, and an ammonium salt represented by a general formula (1) below, in which a concentration of the ammonium salt is 1 g/L or more and 45 g/L or less.

NR₄ ⁺.X⁻  General formula (1)

In the general formula (1), R each represent a hydrogen atom or an alkyl group which may have a side chain and which has 15 or less carbon atoms, X represents a halogen atom, and R may be the same or different from each other.

The aluminum plating solution according to (1) above is an aluminum plating solution that has a wide current density range in which plating can be performed and that has a low solution resistance. Therefore, by using the aluminum plating solution, aluminum plating can be efficiently performed at a low cost.

(2) An aluminum plating solution according to an embodiment of the present invention is the aluminum plating solution according to (1) above, in which the ammonium salt is a dimethylamine hydrochloride or a tetramethylamine hydrochloride, or a mixture of the dimethylamine hydrochloride and the tetramethylamine hydrochloride.

The aluminum plating solution according to (2) above is a plating solution having a lower solution resistance, and thus the cost of the electric power necessary for forming an aluminum plating film can be reduced.

(3) An aluminum plating solution according to an embodiment of the present invention is the aluminum plating solution according to (1) or (2) above, in which the alkylimidazolium halide is 1-ethyl-3-methylimidazolium chloride (EMIC).

According to the aluminum plating solution according to (3) above, the current density in the plating bath is increased, and an aluminum film with a good quality can be efficiently obtained at a higher speed.

(4) An aluminum film according to an embodiment of the present invention is an aluminum film obtained by using the aluminum plating solution according to any one of (1) to (3) above.

Since the aluminum film according to (4) above is obtained by using the aluminum plating solution according to any one of (1) to (3) above, the aluminum film can be efficiently obtained at a high speed at a low cost.

(5) A resin structure according to an embodiment of the present invention is a resin structure including a resin molded body having a three-dimensional network structure and subjected to a conductive treatment, and an aluminum film disposed on a surface of a skeleton of the resin molded body, the aluminum film being obtained by using the aluminum plating solution according to any one of (1) to (3) above.

A porous aluminum object can be manufactured by removing a resin from the resin structure according to (5) above. Since the aluminum film formed on the surface of the skeleton of the resin structure is manufactured with a high efficiency at a low cost, a porous aluminum object can be manufactured with a high efficiency at a low cost by using the resin structure.

(6) A porous aluminum object according to an embodiment of the present invention is a porous aluminum object obtained by removing a resin from the resin structure according to (5) above.

The porous aluminum object according to (6) above is a porous aluminum object that can be efficiently manufactured at a low cost.

(7) A porous aluminum object manufacturing method according to an embodiment of the present invention is a porous aluminum object manufacturing method including a step of performing a conductive treatment on a surface of a skeleton of a resin molded body having a three-dimensional network structure, a step of electrodepositing an aluminum film, using the aluminum plating solution according to any one of (1) to (3) above, on the surface of the skeleton of the resin molded body subjected to the conductive treatment to form a resin structure, and a step of removing a resin from the resin structure.

According to the porous aluminum object manufacturing method according to (7) above, a porous aluminum object having a three-dimensional network structure can be efficiently manufactured at a low cost.

DETAILS OF EMBODIMENT OF THE PRESENT INVENTION

Specific examples of an aluminum plating solution etc. according to embodiments of the present invention will be described below. Note that the present invention is not limited to these exemplifications but defined by the claims, and is intended to include the meaning of equivalents of the claims and all modifications within the scope of the claims.

<Aluminum Plating Solution>

An aluminum plating solution according to an embodiment of the present invention contains an aluminum halide, at least one selected from the group consisting of an alkylimidazolium halide, an alkylpyridinium halide, and a urea compound, and an ammonium salt represented by the general formula (1) below, in which a concentration of the ammonium salt is 1 g/L or more and 45 g/L or less.

NR₄ ⁺.X⁻  General formula (1)

In the general formula (1), R each represent a hydrogen atom or an alkyl group which may have a side chain and which has 15 or less carbon atoms, X represents a halogen atom, and R may be the same or different from each other.

The aluminum plating solution may contain other components as long as the quality of an aluminum film obtained by using the plating solution is not impaired. Specifically, the aluminum plating solution may contain, for example, organic compounds such as xylene, benzene, toluene, and 1,10-phenanthroline.

The aluminum plating solution has a high electrical conductivity, and aluminum plating at a low voltage can be realized even when the plating is performed at a high current density. Accordingly, an increase in the liquid temperature due to an increase in the voltage is also suppressed, and the electric power necessary for operating an apparatus for cooling the plating solution can be reduced. Furthermore, aluminum can be plated at a high speed. Furthermore, by using the aluminum plating solution, an aluminum plating film having a uniform thickness can be formed even in the case where a base having a very complex shape, for example, a resin molded body having a three-dimensional network structure is used.

The term “ammonium salt” is a generic term that represents a chloride of a halogen ion and at last one ion selected from ammonium (NH₄ ⁺), primary ammonium (NRH₃ ⁺), secondary ammonium (NRH₂ ⁺), tertiary ammonium (NR₂H⁺), and quaternary ammonium (NR₄ ⁺). In the ammonium salts, that is, in the general formula (1), R each represent a hydrogen atom or an alkyl group which may have a side chain and which has 15 or less carbon atoms. Among the alkyl groups having 15 or less carbon atoms, a methyl group, an ethyl group, an octyl group, etc. are preferable. Ammonium halide salts having these alkyl groups are preferable from the viewpoint of a high effect of improving the electrical conductivity of the aluminum plating solution.

In the general formula (1), X is not particularly limited as long as X is a halogen atom. However, from the viewpoint of stability of the plating solution, X is preferably chlorine (Cl), bromine (Br), or iodine (I).

The ammonium salt is preferably a dimethylamine hydrochloride or a tetramethylamine hydrochloride, or a mixture of the dimethylamine hydrochloride and the tetramethylamine hydrochloride. These ammonium salts are preferable because the salts have a high effect of improving the electrical conductivity of the aluminum plating solution and are easily available.

A concentration of the ammonium salt in the aluminum plating solution is 1 g/L or more and 45 g/L or less. When the concentration of the ammonium salt is less than 1 g/L, the electrical conductivity of the aluminum plating solution may not be sufficiently improved. When the concentration of the ammonium salt exceeds 45 g/L, the electrical conductivity of the aluminum plating solution improves. However, when an aluminum plating film is formed using the aluminum plating solution, inclusion of the ammonium salt in the resulting aluminum plating film may occur, and an aluminum plating film having a high purity is not obtained.

From these viewpoints, the concentration of the ammonium salt in the aluminum plating solution is more preferably 10 g/L or more and 36 g/L or less, and still more preferably 15 g/L or more and 27 g/L or less.

Examples of the aluminum halide contained in the aluminum plating solution include aluminum chloride (AlCl₃), aluminum bromide (AlBr₃), and aluminum iodide (AlI₃).

The alkylimidazolium halides contained in the aluminum plating solution preferably have an alkyl group having 1 to 5 carbon atoms. Specific examples thereof include 1-ethyl-3-methyl-imidazolium chloride (EMIC) and 1-butyl-3-methyl-imidazolium chloride (BMIC).

The alkylpyridinium halides contained in the aluminum plating solution preferably have an alkyl group having 1 to 5 carbon atoms. Specific examples thereof include butylpyridinium chloride (BPC) and methylpyridinium chloride (MPC).

Among these, an ionic liquid obtained by mixing aluminum chloride and 1-ethyl-3-methyl-imidazolium chloride is preferable because the ionic liquid has good liquid properties.

A mixing ratio of the aluminum halide and the alkylimidazolium halide, the alkylpyridinium halide, or the alkylimidazolium halide and the alkylpyridinium halide is preferably 1:1 or more and 3:1 or less, and more preferably 3:2 or more and 2:1 or less.

The aluminum plating solution may also be obtained by mixing the aluminum halide and a urea compound. The alkylimidazolium halides and the alkylpyridinium halides are relatively expensive organic chlorides. In contrast, the urea compounds are inexpensive and easily available, and thus the aluminum plating solution is easily prepared.

The urea compounds cover urea and derivatives thereof and are not particularly limited as long as the urea compounds form a liquid when mixed with aluminum chloride. For example, compounds represented by the general formula (2) below are preferably used.

In the general formula (2), R each represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or a phenyl group, and R may be the same or different from each other.

Among the above urea compounds, urea and dimethylurea can be particularly preferably used.

An ionic liquid at room temperature can be formed by mixing the urea compound and an aluminum halide. The urea compounds are cheaper and more easily available than the alkylimidazolium halides and the alkylpyridinium halides. Therefore, the aluminum plating solution can be manufactured at a low cost.

A mixing ratio of the urea compound and the aluminum chloride is preferably urea compound:aluminum chloride=1:1.10 to 1:1.50 by molar ratio. When the mixing ratio of aluminum chloride is 1.10 or more, the viscosity of the ionic liquid to be formed can be controlled in a suitable range, and plating can be performed efficiently at a sufficient current density. When the mixing ratio of aluminum chloride is 1.50 or less, mixing of impurities such as a chloride is suppressed in an aluminum film formed on a resin surface, and thus an aluminum film having a good quality can be obtained. Considering the plating efficiency, the amount of aluminum chloride mixed is preferably large. However, aluminum chloride has high corrosiveness, and thus it is not preferable to use an excessively large amount of aluminum chloride.

The mixing ratio of the urea compound and aluminum chloride is more preferably urea compound: aluminum chloride=1:1.10 to 1:1.20, and most preferably 1:1.13 to 1:1.17 by molar ratio.

In addition, it was found that when the mixing ratio of aluminum chloride is 1.13 to 1.17, and in particular, 1.15, the electrical resistance of the plating bath is significantly reduced. It is preferable to control the mixing ratio of aluminum chloride in this range because the voltage necessary for electrodeposition of aluminum can be lowered, which can contribute to energy saving and a reduction in the cost. Furthermore, since an increase in the temperature of the plating solution during operation is also suppressed, the above mixing ratio is also advantageous when the liquid temperature is kept constant.

<Aluminum Film>

An aluminum film according to an embodiment of the present invention is obtained by allowing aluminum to act as an anode and allowing a conductive base to act as a cathode in the aluminum plating solution according to an embodiment of the present invention to electrodeposit aluminum on a surface of the conductive base. As described above, the aluminum plating solution has good electrical conductivity, and an increase in the liquid temperature of the aluminum plating solution during operation is small. Accordingly, the aluminum film is obtained with a high efficiency at a low cost.

<Resin Structure>

A resin structure according to an embodiment of the present invention is obtained by allowing aluminum to act as an anode and allowing a resin molded body having a three-dimensional network structure and subjected to a conductive treatment to act as a cathode in the aluminum plating solution to electrodeposit aluminum on a surface of a skeleton of the resin molded body.

The resin molded body and a method for performing a conductive treatment on the resin molded body will be specifically described below.

Any resin can be selected as the material of the resin molded body. Examples of the material include foamed resin molded bodies prepared by using polyurethane, melamine, or the like. Although the examples of the material are described as “foamed resin molded bodies”, resin molded bodies having any shape may be selected as long as the resin molded bodies have communicating pores (continuous pores). For example, non-woven fabric-like materials obtained by intertwining fibrous resins such as polypropylene or polyethylene may also be used instead of the foamed resin molded bodies.

The resin molded body preferably has a porosity of 80% to 98% and a pore diameter of 50 to 500 μm. Urethane foams and melamine foams can be preferably used as the resin molded bodies because they have a high porosity, pores thereof have continuity, and they have good thermal degradability. Urethane foams are preferable from the viewpoint of the uniformity of pores, availability, etc., and from the viewpoint that a resin molded body having a small pore diameter is obtained. Foamed resin molded bodies formed of a urethane foam, a melamine foam, or the like often contain residues such as a foam stabilizer used in a foaming step and an unreacted monomer, and thus are preferably subjected to a washing treatment in advance.

The porosity of the resin molded body is defined by the following formula.

Porosity=(1−(weight of porous material [g]/(volume of porous material [cm³]×density of raw material))×100[%]

The pore diameter is determined by magnifying a surface of the resin molded body by means of a photomicrograph or the like, counting the number of pores per inch (25.4 mm) as a cell number, and calculating an average value as mean pore diameter=25.4 mm/cell number.

The conductive treatment of a resin surface can be selected from methods including know methods. It is possible to employ a method including forming a metal layer composed of nickel or the like by electroless plating or a gas-phase method, and a method including forming a metal or carbon layer using a conductive coating material.

By forming a metal layer on a resin surface by electroless plating or a gas-phase method, the electrical conductivity of the resin surface can be increased. On the other hand, although the conductive treatment of a resin surface by carbon coating is somewhat poor from the viewpoint of electrical conductivity, this conductive treatment can be performed so that a metal other than aluminum is not mixed in the resulting aluminum structure after plating. Accordingly, a structure that is substantially composed of only aluminum as a metal can be manufactured. This method is also advantageous in that electrical conductivity can be imparted at a low cost.

The conductive treatment can be performed by applying a carbon coating material to a surface of the skeleton of the resin molded body. A suspension serving as the carbon coating material used in this case preferably contains a binder, a dispersant, and a dispersion medium, in addition to carbon particles.

In order to uniformly apply carbon particles to the surface of the skeleton of the resin molded body, it is necessary that the suspension maintain a uniform suspension state. For this purpose, the suspension is preferably maintained at 20° C. to 40° C. By maintaining the temperature of the suspension at 20° C. or higher, a uniform suspension state can be maintained, it is possible to prevent only the binder from concentrating on the surface of the skeleton that forms a network structure of the resin molded body and forming a layer, and thus the carbon particles can be uniformly applied. Since the layer of carbon particles that are uniformly applied in this manner is not easily separated, aluminum plating that strongly adheres to the layer can be formed. On the other hand, since the temperature of the suspension is 40° C. or lower, evaporation of the dispersant can be suppressed. Accordingly, it is possible to suppress the phenomenon in which the suspension is not easily concentrated with the coating process time.

The carbon particles have a particle size of 0.01 to 5 μm, and preferably 0.01 to 0.5 μm. When the particle size is excessively large, the carbon particles may clog pores of the resin molded body or inhibit smooth plating. When the particle size is excessively small, it becomes difficult to ensure sufficient electrical conductivity.

<Porous Aluminum Object>

A porous aluminum object according to an embodiment of the present invention is a porous aluminum object obtained by removing a resin from the resin structure according to an embodiment of the present invention. The porous aluminum object has a three-dimensional network structure and can exhibit good characteristics when applied to various filters, catalyst supports, and battery electrodes, etc.

The method for removing a resin from the resin structure is not particularly limited. For example, the resin is burned away by performing heat treatment in which the resin structure is heated in a nitrogen atmosphere or an air atmosphere at 370° C. or higher, at which the resin is decomposed, preferably 500° C. or higher. Consequently, a porous aluminum object can be obtained.

<Porous Aluminum Object Manufacturing Method>

A porous aluminum object manufacturing method according to an embodiment of the present invention includes a step of performing a conductive treatment on a surface of a skeleton of a resin molded body having a three-dimensional network structure, a step of electrodepositing an aluminum film, using the aluminum plating solution according to an embodiment of the present invention, on the surface of the skeleton of the resin molded body subjected to the conductive treatment to form a resin structure, and a step of removing a resin from the resin structure.

Step of Performing Conductive Treatment on Resin Molded Body

This step is performed for imparting electrical conductivity to a resin molded body having a three-dimensional network structure by forming an electrically conductive layer on a surface of a skeleton on the resin molded body. The resin molded body described above can be preferably used as the resin molded body having a three-dimensional network structure. The conductive treatment of the resin molded body may also be performed as described above.

Step of Forming Aluminum Plating Film

In order to form an aluminum film on the surface of the skeleton of the resin molded body that has been subjected to the conducive treatment, as described above, aluminum is allowed to act as an anode and the resin molded body having the three-dimensional network structure and subjected to the conductive treatment is allowed to act as a cathode in the aluminum plating solution according to an embodiment of the present invention. Consequently, aluminum is electrodeposited on the surface of the skeleton of the resin molded body, and the resin structure can be obtained.

Step of Removing Resin

In order to remove a resin from the resin structure, as described above, heat treatment is performed in a nitrogen atmosphere, an air atmosphere, or the like.

EXAMPLES

The present invention will be described in more detail on the basis of Examples. These Examples are only illustrative, and an aluminum plating solution etc. of the present invention are not limited thereto. It is to be noted that the scope of the present invention is defined by the claims and includes the meaning of equivalents of the claims and all modifications within the scope of the claims.

EXAMPLE 1

Aluminum chloride and 1-ethyl-3-methylimidazolium chloride (EMIC) were mixed so that the molar ratio became 2:1 to prepare an ionic liquid 1. Methylammonium chloride, which is a primary ammonium salt, was further added to the ionic liquid 1 so as to have a concentration of 15 g/L. Thus, an aluminum plating solution 1 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 1 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 1 was increased by 11%. The electrical conductivities were measured by an alternating current impedance method.

EXAMPLE 2

Dimethylamine hydrochloride, which is a secondary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 27 g/L. Thus, an aluminum plating solution 2 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 2 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 2 was increased by 20%.

EXAMPLE 3

Trimethylammonium chloride, which is a tertiary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 21 g/L. Thus, an aluminum plating solution 3 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 3 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 3 was increased by 13%.

EXAMPLE 4

Tetraoctylammonium chloride, which is a quaternary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 18 g/L. Thus, an aluminum plating solution 4 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 4 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 4 was increased by 18%.

EXAMPLE 5

Ammonium chloride was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 10 g/L. Thus, an aluminum plating solution 5 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 5 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 5 was increased by 5%.

EXAMPLE 6

Dimethylamine hydrochloride, which is a secondary ammonium salt, and tetramethylammonium chloride, which is a quaternary ammonium salt, were further added to the ionic liquid 1 prepared in Example 1 so that the concentrations of dimethylamine hydrochloride and tetramethylammonium chloride were 15 g/L and 10 g/L, respectively. Thus, an aluminum plating solution 6 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 6 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 6 was increased by 22%.

EXAMPLE 7

Aluminum chloride and 1-butylpyridinium chloride (BPC) were mixed so that the molar ratio became 2:1 to prepare an ionic liquid 2. Tetramethylammonium chloride, which is a quaternary ammonium salt, was further added to the ionic liquid 2 so as to have a concentration of 18 g/L. Thus, an aluminum plating solution 7 was prepared.

Electrical conductivities of the ionic liquid 2 and the aluminum plating solution 7 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 7 was increased by 15%.

EXAMPLE 8

Aluminum chloride and urea were mixed so that the molar ratio became 1.5:1 to prepare an ionic liquid 3. Dimethylamine hydrochloride, which is a secondary ammonium salt, was further added to the ionic liquid 3 so as to have a concentration of 27 g/L. Thus, an aluminum plating solution 8 was prepared.

Electrical conductivities of the ionic liquid 3 and the aluminum plating solution 8 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 8 was increased by 25%.

EXAMPLE 9

Aluminum chloride, EMIC, BPC, and urea were mixed so that the molar ratio became 2:0.8:0.1:0.1 to prepare an ionic liquid 4. Tetramethylammonium chloride, which is a quaternary ammonium salt, was further added to the ionic liquid 4 so as to have a concentration of 15 g/L. Thus, an aluminum plating solution 9 was prepared.

Electrical conductivities of the ionic liquid 4 and the aluminum plating solution 9 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 9 was increased by 18%.

EXAMPLE 10

Dimethylamine hydrochloride, which is a secondary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 1 g/L. Thus, an aluminum plating solution 10 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 10 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 10 was increased by 3%.

EXAMPLE 11

Dimethylamine hydrochloride, which is a secondary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 45 g/L. Thus, an aluminum plating solution 11 was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution 11 were measured and compared. According to the results, it was confirmed that the electrical conductivity of the aluminum plating solution 11 was increased by 15%.

EXAMPLE 12

A porous aluminum object was prepared as follows using the aluminum plating solution 1 prepared in Example 1.

Formation of Electrically Conductive Layer

As a resin molded body having a three-dimensional network structure, a urethane foam having a thickness of 1 mm, a porosity of 95%, and a number of pores (cell number) per inch of about 46 was prepared and cut into a rectangular shape of 50 mm×80 mm. The urethane foam was immersed in a carbon suspension and dried. An electrically conductive layer that included carbon particles adhering thereto was thereby formed over the entire surface of the urethane foam. The carbon suspension contained, as components, a mixture of graphite and carbon black in an amount of 25%, a resin binder, a penetrant, and an antifoaming agent. The carbon black had a particle size of 0.5 μm.

Molten Salt Plating

Subsequently, aluminum was electrodeposited, using the aluminum plating solution 1 prepared in Example 1, on the surface of the skeleton of the above-prepared urethane foam subjected to the conductive treatment to obtain a resin structure. Regarding plating conductions, plating was performed in the plating solution at a current density of 6.0 A/dm² while stirring the plating solution. The stirring was performed with a stirrer using a rotator made of Teflon (registered trademark). Note that the current density is a value calculated on the basis of the apparent area of the urethane foam. The amount of the aluminum plating solution 1 was 0.5 L.

Removal of Resin

The resin structure obtained as described above was taken from the plating bath, subjected to a water-washing treatment, and then heat-treated in air at 600° C. for 30 minutes. Through this step, the resin was burned away to obtain a porous aluminum object (purity 99.9% by mass).

(Evaluation)

In the step of electrodepositing aluminum on the surface of the skeleton of the resin molded body, that is, in the aluminum plating step, an increase in the liquid temperature of the plating solution was about 10° C. In the case where plating was performed using the ionic liquid 1, the increase in the liquid temperature was 30° C. Comparing these results, it was confirmed that a load for controlling the liquid temperature was reduced, and the electric power cost can thereby also be reduced. Furthermore, the plating film was formed uniformly.

COMPARATIVE EXAMPLE 1

Dimethylamine hydrochloride, which is a secondary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 0.5 g/L. Thus, an aluminum plating solution A was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution A were measured and compared. According to the results, the electrical conductivity of the aluminum plating solution A did not substantially increase, that is, a rate of increase in the electrical conductivity was 0.2%.

COMPARATIVE EXAMPLE 2

Dimethylamine hydrochloride, which is a secondary ammonium salt, was further added to the ionic liquid 1 prepared in Example 1 so as to have a concentration of 47 g/L. Thus, an aluminum plating solution B was prepared.

Electrical conductivities of the ionic liquid 1 and the aluminum plating solution B were measured and compared. According to the results, the electrical conductivity of the aluminum plating solution B was increased by 10%. However, dimethylamine hydrochloride was included in the resulting plating film, and the purity of the aluminum plating film was 99.8%, which was not preferable. 

1. An aluminum plating solution comprising: an aluminum halide; at least one selected from the group consisting of an alkylimidazolium halide, an alkylpyridinium halide, and a urea compound; and an ammonium salt represented by a general formula (1), wherein a concentration of the ammonium salt is 1 g/L or more and 45 g/L or less: NR₄ ⁺.X⁻  General formula (1) where, in the general formula, R each represent a hydrogen atom or an alkyl group which may have a side chain and which has 15 or less carbon atoms, X represents a halogen atom, and R may be the same or different from each other.
 2. The aluminum plating solution according to claim 1, wherein the ammonium salt is a dimethylamine hydrochloride or a tetramethylammonium chloride, or a mixture of the dimethylamine hydrochloride and the tetramethylammonium chloride.
 3. The aluminum plating solution according to claim 1, wherein the alkylimidazolium halide is 1-ethyl-3-methylimidazolium chloride (EMIC).
 4. An aluminum film obtained by using the aluminum plating solution according to claim
 1. 5. A resin structure comprising a resin molded body having a three-dimensional network structure and subjected to a conductive treatment; and an aluminum film disposed on a surface of a skeleton of the resin molded body, the aluminum film being obtained by using the aluminum plating solution according to claim
 1. 6. A porous aluminum object obtained by removing a resin from the resin structure according to claim
 5. 7. A porous aluminum object manufacturing method comprising: a step of performing a conductive treatment on a surface of a skeleton of a resin molded body having a three-dimensional network structure; a step of electrodepositing an aluminum film, using the aluminum plating solution according to claim 1, on the surface of the skeleton of the resin molded body subjected to the conductive treatment to form a resin structure; and a step of removing a resin from the resin structure. 